The present invention relates to a copper-containing conductor composition, and more particularly to a composition for providing a thick film copper conductor which is formed into conductor patterns or electrodes on, mainly, ceramic substrates.
Thick film conductor compositions are composed of, generally, a conductive metal, an inorganic oxide and a glass powder as an inorganic bonding agent, which are in finely divided form and are dispersed in an organic medium. The conductive metal which is one ingredient in the thick film conductor composition is ordinarily gold, silver, palladium, platinum, or mixtures and alloys thereof. However, these metals are noble metals, and they are expensive and have large fluctuations in price. Accordingly, it has been strongly expected to substitute base metals, particularly copper metal, which are cheap and have small fluctuations in price, for the noble metals.
As to the copper-containing thick film conductor composition, Japanese Examined Patent Publication No. 2398/1984 discloses a composition composed of a copper powder and a copper oxide powder, and a lead oxide powder and/or a bismuth oxide powder, which does not require the use of a glass binder. In this system, however, the copper, that is, the surface of the copper thick film conductor is oxidized by oxygen which is introduced in a trace amount into a firing atmosphere, usually a nitrogen atmosphere, in order to decompose or volatilize organic matters, thus resulting in deterioration of solderability (wettability by solder). Japanese Unexamined Patent Publication No. 70746/1985 discloses a composition composed of a copper powder and an inorganic binder, and tungsten, molybdenum or rhenium, and teaches that the solderability is improved by utilizing the property of tungsten, molybdenum or rhenium to catch oxygen. These metals are, however, reacted with oxygen to produce metal oxides capable of subliming or volatile metal oxides, thus often resulting in contamination of the inside of a firing furnace or deposition of the metal oxides on a circuit.
U.S. Pat. No. 4,521,329 to Siuta et al is directed to copper conductor compositions comprising metallic copper particles, a specific amount of copper oxide particles and an inorganic binder, which are dispersed in an organic medium. The patent discusses in its Example that zinc oxide is used in a small amount as one ingredient of the glass frit, but it is silent with respect to the use of a powder of zinc oxide per se as the inorganic binder. Zinc oxide contained in the glass frit is not in the state of particles but is an ingredient of the glass frit, and is bulkily and uniformly distributed, so that it cannot be formed into a barrier layer or zone capable of preventing to bleed the glass frit and exists thinly in the copper conductor, therefore the zinc oxide contained in the glass frit as one ingredient cannot prevent the bleeding of the glass frit, resulting in not only poor solderability but also deterioration of adhesion strength after aging.
An object of the present invention is to improve the defects of conventional copper thick film conductors, that is, the poor solderability and poor adhesive strength to substrates, particularly deterioration of adhesive strength to substrates in various aging tests.
This and other objects of the present invention will become apparent from the description hereinafter.